Cook noodles with onion following package directions; drain. Place noodles and onion in 4-quart casserole. Fold in cubed pot roast along with any gravy.
Whisk together soup, milk, sour cream, lemon juice, salt and pepper in a bowl. Pour over noodles; stir to combine completely.
Bake in 350°F oven 30 minutes. Let cool 10 minutes before serving.